1. Field of the Invention
This invention relates to shaping sheets of deformable materials, such as glass sheets, and particularly relates to the high speed production of bent glass sheets that are toughened by air quenching, and most particularly, for shaping and heat treating relatively thin glass sheets, particularly those having a nominal thickness of 1/8 inch (3.2 mm) or less. Thinner glass sheets sag more readily than thicker glass sheets at any given elevated temperature above the glass deformation temperature. Hence, it is more difficult to control the shape imparted to thinner glass sheets.
Shaped and tempered glass sheets are widely used as side windows or rear windows in vehicles such as automobiles or the like and, to be suitable for such application, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the frames defining the window openings into which the glass side or rear windows are installed. It is also important that the side or rear windows meet stringent optical requirements and that the windows be free of optical defects that would tend to interfere with the clear viewing therethrough in their viewing areas.
During fabrication, glass sheets intended for use as shaped windows in vehicles are subject to thermal treatment to temper the glass for strengthening the glass and increasing the resistance of the shaped window to damage resulting from impact. In addition to increasing the resistance of a glass sheet to breakage, tempering also causes a glass sheet to fracture into relatively small, relatively smoothly surfaced fragments that are less injurious than the relatively large, jagged fragments that result from the more frequent breakage of untempered glass.
The commercial production of shaped glass sheets for such purposes commonly includes heating flat sheets to the softening point of the glass, shaping the heated sheets to a desired curvature and then cooling the bent sheets in a controlled manner to a temperature below the annealing range of the glass. During such treatment, a glass sheet is conveyed along a substantially horizontal path that extends through a tunnel-type furnace where the glass sheet is one of a series of sheets that are heated to the deformation temperature of glass and along an extension of said path into a shaping station (located either within the furnace or immediately beyond the furnace) where each glass sheet in turn is transferred onto a vacuum holder. The vacuum holder holds the heat-softened glass sheet thereagainst by suction. At about the same time, a transfer and tempering ring having an outline shape conforming to that desired for the glass sheet slightly inboard of its perimeter moves upstream into a position below the vacuum holder. Release of the vacuum deposits the glass sheet onto the tempering ring. The tempering ring supports the peripheral portion of the glass sheet while it conveys the glass sheet into a cooling station for rapid cooling.
In prior art apparatus, the vacuum mold was either provided with a lower, rigidly curved shaping surface that shaped the heat-softened glass sheet incrementally by suction thereagainst or had a smoothly surfaced flat shaping surface that lifted the flat glass sheet by suction thereagainst and depended on a release of the vacuum within the mold to permit the hot glass sheet to drop by gravity or by replacing the vacuum with positive pressure to drop the glass sheet by a combination of gravity and an additional force onto the tempering ring to develop the shape dictated by the outline configuration of the tempering ring. The latter process has been called drop forming.
When a rigid, curved surface is adjacent a heat-softened flat glass sheet during the application of suction through said surface, much power is needed to obtain the suction necessary to lift and shape a hot glass sheet simultaneously by suction at a rate sufficiently rapid to provide a high speed mass production operation for shaping and tempering glass sheets. The glass sheet bending art has developed distortable vacuum holders that normally have a smooth, flat, lower perforated surface and engage the upper surface of one or more flat glass sheets by suction and distort the engaged heat-softened glass sheeets to a desired shape as they lift the engaged glass sheets. A ring-like member having both an outline shape and elevational configuration desired for the glass sheet shuttles to a position below the vacuum holder. When vacuum is either stopped or replaced by a positive downward pressure, the glass sheet drops onto the ring-like member for conveyance thereon to a cooling station where the hot bent glass sheet is cooled sufficiently rapidly to impart a temper to the sheet.
Vacuum holders having a plurality of vacuum chambers with a unique level of vacuum for each vacuum chamber are provided with a cover of fibrous material. Consequently, when vacuum is applied to the different chambers of the vacuum holder at different vacuum leels, the residual air in one vacuum chamber tends to bleed into an adjacent vacuum chamber through the porous passages provided by the porous fiber glass cover. As a result, a higher level of vacuum must be applied to the vacuum chambers subjected to a relatively high level of vacuum to compensate for the loss of vacuum level resulting from such tendency to bleed. The level of vacuum is one of several factors that tends to determine the severity that the pattern of the porous cover is replicated on the surface of a glass sheet shaped with apparatus employing a vacuum holder. Such replication causes surface marks that makes the bent glass less desirable as a commercial item.
2. Description of Patents of Interest
U.S. Pat. No. 4,277,276 to John D. Kellar and Gordon F. Pereman discloses glass sheet shaping and tempering apparatus that comprises a deformable vacuum holder having a glass engaging surface as smooth as that of flat vacuum molds of the prior art, and that also shapes the glass sheet to a shape approximately its final desired shape adjacent a heating furnace before releasing the glass sheet onto a shaping and tempering ring to make it possible to increase the speed of a mass production operation for shaping and tempering glass sheets, particularly those thinner than 3.2 mm nominal thickness.
U.S. Pat. No. 4,282,026 to Harold A. McMaster, Norman C. Nitschke and John S. Nitschke discloses bonding a board of fibrous material to a rigid vacuum holder of ceramic composition in glass sheet bending apparatus that uses a vacuum holder in the glass shaping process. The adhesive bond is not durable enough to adhere a board of fibrous material to a deformable metal sheet surface at elevated temperature cycles required to shape glass sheets against a deformable vacuum holder.
Whenever thin glass sheets are shaped within a heating furnace by sandwiching between pressing molds of complementary shape, such a process saves energy because it avoids the need to overheat the glass, which is necessary to overcome the rapid cooling of the glass sheet that takes place en route to a shaping station located outside the furnace. However, it is difficult to control the shape and temperature of vacuum molds permanently installed within a furnace. It is also difficult to obtain access to repair or to provide maintenance for a mold that is permanently installed within a furnace.
U.S. Pat. No. 4,297,118 to John D. Kellar and Gordon F. Pereman provides a glass sheet shaping method using apparatus that comprises an upper vacuum holder that first engages and lifts a heat-softened glass sheet by suction, and that provides clearance for a shaping and tempering ring to enter the shaping station. The glass sheet shaping station of this patent is located within the heating furnace. The vacuum mold that is used to engage and lift a heat-softened glass sheet by suction is moved outside the furnace between successive bending operations. Such movement cools the vacuum mold intermittently. Such intermittent cooling helps to control the temperature rise somewhat and consequently the shape of the vacuum mold departs only a limited amount from its desired shape during a mass production campaign that uses this patented invention.
U.S. Pat. No. 4,349,375 to John D. Kellar and Gordon F. Pereman discloses other apparatus that comprises a more durable, deformable vacuum holder capable of assuming a flat configuration suitable for engaging a flat glass sheet by suction and deforming into a curved configuration. Despite the utility of apparatus of this type that has been demonstrated in commercial operations, room exists for further improvement in results obtained using vacuum holders, such as providing vacuum holders that require less frequent maintenance during elevated cyclic temperature conditions associated with glass sheet shaping, reducing bulging of the vacuum holders to limits acceptable to the customer, providing even more uniform, repetitive deformation of the vacuum holder than before, avoiding collapse of the vacuum holder, providing more efficient cooling of the holder between shaping cycles, avoiding misalignment of glass sheets with the distorted shape of the vacuum holder and helping maintain different levels of vacuum in adjacent vacuum chambers of multiple chamber vacuum holders.